Combined heat and power system

ABSTRACT

A combined heat and power system comprising a Stirling engine ( 1 ) the head ( 2 ) of which is heated by a burner. A heat exchanger ( 10 ) absorbs heat from exhaust gases from the burner which have heated the engine head. A supplementary burner ( 22, 23 ) generates additional heat which is directly absorbed by the heat exchanger. The supplementary burner is a multi-stage burner with separate stages ( 22, 23 ) which are independently controlled.

The present invention relates to a combined heat and power system. Inparticular, the present invention relates to a combined heat and powersystem comprising a Stirling engine having a head; a burner to heat theStirling engine head so that the Stirling engine can generate electricalenergy; a heat exchanger arranged to absorb heat from exhaust gases fromthe burner which have heated the engine head; and a supplementary burnerto generate additional heat which is directly absorbed by the heatexchanger.

Such a combined heat and power system will subsequently be referred toas “of the kind described”.

A system of the kind described is known for use in a domesticenvironment as a domestic combined heat and power (dchp) system. TheStirling engine supplies some of the domestic electrical powerrequirement with the remainder being supplied from the mains. The heatoutput from the Stirling engine supplies some of the domestic heat loadwith the remainder being supplied by the supplementary burner. TheStirling engine burner and supplementary burner are modulated to providethe heat output required by the home and the system is controlled toallow the Stirling engine to generate for as great a proportion of thetime as is possible.

There are, however, limits on the range of heat output that can beobtained from the burners due to minimum flow requirements through each.This results in a step change in heat output when the engine burnercapacity is exceeded. The combined supplementary and engine burners,both operating at minimum settings, will produce a higher heat outputthan the engine burner alone produces at its maximum. This will resultin a lack of control of heat output in the operating region in between.As this has been found to be a critical operating area for the domesticheating system, this problem could result in a reduction in user comfortand system efficiency.

According to the present invention, in a system of the kind described,the supplementary burner is a multi-stage burner with separate stageswhich are independently controllable.

By having multiple independently controllable stages, more exact controlcan be provided of the heat output across the full range required.

Greater flexibility is provided if at least one stage has a differentheat output range from that of another stage. In practice, the range ofheat output of each stage can be made different.

In order to introduce further flexibility still, at least one of thesupplementary burner stages may be arranged to heat both the heatexchanger directly and the Stirling engine head. Thus, when this stageis being fired without the engine burner being fired, the Stirlingengine will operate at a reduced electrical output. While when thisstage is being operated together with the engine burner, the Stirlingengine will operate at peak electrical output.

The supplementary burner may be positioned anywhere provided that it canprovide adequate heat to the heat exchanger. However, preferably, thesupplementary burner is radially outward of the heat exchanger as thisprovides for more convenient packaging.

The heat exchanger may have any known construction, for example it canbe provided by a duct surrounded by a water jacket. However, preferably,the heat exchanger comprises a helical coil or several helical coilsconnected in series for a heat exchanger fluid wound around an axis andwhich extends along the full axial length of the supplementary burner.This provides an efficient way of packaging a multi-stage burner asthose coils which are adjacent to the burner stage(s) which is/are beingfired will receive heat in an efficient manner.

Also, with the helical coil arrangement, part of the coil can bearranged to surround part of the Stirling engine head to provide thesupplementary burner stage which is arranged to heat the Stirling enginehead and the heat exchanger directly as referred to above.

The Stirling engine may be mounted with its head uppermost as is wellknown in the art. However, preferably, the Stirling engine is mountedwith its head lowermost and the supplementary burner and heat exchangerbeing positioned directly beneath the head. One benefit of thisarrangement is that it is a simple matter to provide a drain for fluidswhich have condensed out of the combustion gases. Another benefit isthat the engine can be hung from at least one spring attached to its topend. Such a mounting allows easy access to the engine for routinemaintenance and for routing water pipes to and from the engine.

An additional problem encountered in the design of a heat exchanger fora dchp system has been the removal of condensate from the helical tubinginside the supplementary heat exchanger. Our earlier applicationPCT/GB02/05711 discloses a heat exchanger with a tubing helically woundabout a horizontal axis. With such an arrangement some of the condensatecollects on the horizontal areas of the tubing.

It is usual to mount the Stirling engine from resilient mounting toallow the Stirling engine to vibrate. The vibrations can be reduced bymounting an absorber mass resiliently to the engine which can be tunedto vibrate in counter-phase to the Stirling engine thereby reducing theoverall vibration. The ancillary elements of the system such as theburners and heat exchanger are then mounted separately from the Stirlingengine.

However, it has been found that, if the heat exchanger is mounted on theStirling engine, a residual amount of vibration will be passed to theheat exchanger which will aid in removing the condensate.

This aspect of the invention also forms an independent invention whichcan be broadly described as a Stirling engine assembly comprising aStirling engine for generating electrical power and heat output, and aheat exchanger to absorb some of the heat output, the heat exchangerbeing mounted substantially only the Stirling engine so as to vibratetogether with the engine.

The engine must be dynamically balanced. Therefore, preferably, the massof the heat exchanger is balanced about the main axis of the invention.This avoids the need for complex counterbalancing measures.

This construction is particularly effective if the Stirling engine ismounted with its head lowermost and the heat exchanger is mounteddirectly beneath the head.

An engine burner and a supplementary burner may also be mounted to theStirling engine.

Also, preferably, the heat exchanger comprises a helical coil centred onthe main axis of the engine. The Stirling engine has a main axis whichis preferably coaxial with an axis about which the helical coil iswound. By arranging the heat exchanger in this way, there are nohorizontal surfaces, such that a condensate can flow down the pipesaided by the vibration transmitted from the Stirling engine.

An example of the system in accordance with the present invention willnow be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross-section of the system;

FIG. 2 is a schematic perspective view of the valve assembly; and

FIG. 3 is a graphical representation of the valve sleeve orificeprofiles.

FIG. 1 shows the overall Stirling engine assembly. This comprises aStirling engine 1 which is mounted in “inverted” configuration, namelywith the engine head 2 lowermost. The engine is suspended from aplurality of springs 3 attached to a mounting bracket 4 and whichsurrounds the engine 1. Alternatively it could be suspended from asingle centrally located spring. An annular absorption mass 5 surroundsthe engine and is attached thereto by a plurality of resilient mounts 6in order to absorb vibrations of the engine. The engine head 2 isprovided with a plurality of annular fins 7 which absorb heat in amanner to be described thereby heating the engine head. The engine alsohas an engine cooler 8 which is cooled by circulating water again in amanner to be described.

A heat exchanger is provided by a helical coil 10 which is coaxial withthe Stirling engine 1. The coil 10 surrounds the bottom end of the head2 beneath the fins 7, and then extends axially below the engine 1. Thecoil 10 is provided in three distinct stages. The first stage 11surrounds the engine head 2, the second stage 12 is beneath the enginehead 2 and is separated therefrom by a baffle 13 which is perforated atits outer periphery. The third stage 14 is axially below the secondstage 12 and is wound at a smaller radius than the first two stages. Thethird stage 14 is not surrounded by a burner. The baffle 13 maintainsthe temperature and pressure in the first 11 and second 12 stages whenthe third stage 14 is not firing. Also, heat from the first 11 andsecond 12 stages is reflected back onto the head 2 of the engine 1.

The burner 20 has an annular configuration and surrounds the engine head2 and the coil 10. The burner 20 is also divided into stages. A firststage 21 surrounds the engine head 2 and annular fins 7. A second stage22 surrounds the lower end of the engine head 2 and the first stagecoils 11. The third stage 23 of the burner surrounds the second stagecoils 12.

It should also be noted that the second 22 and third 23 burner stagesare each themselves divided into two sub-stages 22A, 22B, 23A, 23B eachof which is independently operable. Each of the first burner stage 21,the second sub-stages 22A, 22B and third sub-stages 23A, 23B has aseparate supply of air and combustible gas along lines 30, 31, 32, 33and 34 respectively. Each of the stages is also provided with its ownignition system and dedicated ignition control such that each can beignited independently. It should also be noted that, elsewhere in thespecification, references are made to “a burner to heat the Stirlingengine” and “a supplementary burner”. These two burners can be part of asingle multi-stage burner as described with reference to FIG. 1.

The control of the gas and air mixture is achieved by valve 40 asdescribed in more detail in FIGS. 2 and 3.

The valve 40 comprises an inner sleeve 41 and outer sleeve 42. For eachoutlet line the inner 41 and outer 42 sleeves have a pair of orifices43, 44 of different shapes which are arranged to overlap to differingdegrees as the inner 41 and outer 42 sleeves are rotated with respect toone another. The flow through each of the lines 30-34 is determined bythe degree of overlap of the two orifices. A solenoid 45 in the lowerend of the housing with the valve 40 provides the relative rotationalmovement between the inner 41 and outer 42 sleeve.

The relative sizes of the orifices 43, 44 are arranged to ensure thatthe flow through each outlet 30-34 is, as closely as possible, a linearfunction of the rotary position of the inner 41 and outer 42 sleeves.

A detailed view of the orifice profiles is given in FIG. 3. The hatchedareas represent the projection of the orifices 44 onto a flat plane.FIG. 3. This shows the positions for which the heat and power output arevariable (VAR), maximum (MAX), minimal (MIN), or zero (NONE). It will beappreciated that independent control of all of the outlet streams is notpossible through all of the lines 30-33 given the fixed relationshipbetween the two sets of orifices 43, 44. However, it will be appreciatedfrom FIG. 1 that true independence of all of the stages of the burner isnot absolutely necessary. For example, for the second stage 22, it willbe necessary either to fire first sub-stage 22A alone or to fire thissub-stage in combination with the second sub-stage 22B. However, it willnot be necessary to fire the second sub-stage 22B alone. Similarconsiderations apply to the third stage 23. If greater independence isrequired the inner sleeve 41 may be separated into a number ofindependently rotatable segments.

In use, the operation of the Stirling engine assembly is determined bythe domestic demand for electricity and heat. If the demand forelectricity is high and the demand for heat is low, the first burnerstage 21 and optionally the second burner stage 22 will be fired.Conversely, if the demand for heat is high and the demand forelectricity is low, the third burner stage 23 and optionally the secondburner stage 22 will be fired. If demand for both electricity and heatis high, all burner stages will be fired. Within these extremes are anumber of intermediate settings provided by the various burner stagesand sub-stages allowing a high degree of control of the electrical powerand heat generation.

Gas and air for the burners is mixed in a fan/Venturi gas valvearrangement 46 and the speed of the fan can be controlled to vary theoverall amount of gas supplied to the various burner stages. Theposition of the inner sleeve 41 of the valve 40 within the outer sleeve42 is then determined according to the system requirements to ensure thecorrect supply of gas and air to the various burner stages.

Alternatively, the gas may be mixed with the air downstream of the valve40. This means that the valve 40 does not have to be sealed to allowpre-mixing. However, it would require a gas/air mixer for each streamfrom the valve 40.

Water from the domestic central heating system is fed into the Stirlingengine system along line 50 where it first cools the engine cooler 8 andhence absorbs low grade heat. It is then circulated around an annularpassage 51 where it cools a seal 52 between the first stage burner 21and the engine housing. Optionally, one or more cooling channels mayfeed the water around the outside casing of the burner. These channelsmay either be in series with or parallel to the passage 51 around theseal 52. The water is then fed along line 53 into the supplementary heatexchanger 10. It initially passes around the third stage 14 in which itpicks up relatively low grade heat as the third stage is not heateddirectly by a burner stage. It then circulates up through the helicalwindings through the second 12 and first 11 windings respectively whereit is heated by whichever burner stages are active at the time beforefinally exiting along line 54 to the domestic central heating system.

The exhaust gas from the various burner stages exits along flue 60,while condensate from the exhaust gases is drained along condensatedrain 61 which is positioned to avoid any dead space where fluid couldbuild up and cause corrosion.

As can be seen in FIG. 1, the heat exchanger coil 10 and burner assembly20 are mounted directly from the Stirling engine 1 via an annularbracket 55. Thus, the small residual vibration of the Stirling engine 1which remains despite the effect of the absorption mass 5 is transmittedto the coils of the supplementary heat exchanger. Any condensation whichsettles on the coils is encouraged by this vibration to flow along thedownwardly angled surfaces of the coil 10 ultimately to the condensatedrain 60.

1. A combined heat and power system comprising a Stirling engine havinga head; a burner to heat the Stirling engine so that the Stirling enginecan generate electrical energy; a heat exchanger arranged to absorb heatfrom exhaust gases from the burner which have heated the engine head;and a supplementary burner to generate additional heat which is directlyabsorbed by the heat exchanger; wherein the supplementary burner is amulti-stage burner with separate stages which are independentlycontrolled.
 2. A system according to claim 1, wherein at least one stagehas a heat output range which is different from that of another stage.3. A system according to claim 1, wherein at least one of thesupplementary burner stages is arranged to heat both the heat exchangerdirectly and the Stirling engine head.
 4. A system according to claim 1,wherein the supplementary burner is radially outward of the heatexchanger.
 5. A system according to claim 3, wherein the heat exchangercomprises a helical coil wound around an axis and which extends alongthe full axial length of the supplementary burner.
 6. A system accordingto claim 5, wherein the helical coil also surrounds part of the Stirlingengine head.
 7. A system according to claim 1, wherein the Stirlingengine is mounted with its head lowermost and the supplementary burnerand heat exchanger are directly beneath the head.
 8. A system accordingto claim 7, wherein the engine is hung from at least one spring attachedto its top end.
 9. A system according to claim 1, wherein the heatexchanger is mounted on the Stirling engine.
 10. A Stirling engineassembly comprising a Stirling engine for generating electrical powerand heat output, and a heat exchanger to absorb some of the heat output,the heat exchanger being mounted substantially only on the Stirlingengine so as to vibrate together with the engine.
 11. An assemblyaccording to claim 10, wherein the engine has a main axis ofreciprocation and the mass of the heat exchanger is balanced about themain axis of the engine.
 12. An assembly according to claim 10, whereinthe Stirling engine has a head which is heated, in use, and furthercomprising a burner to heat the head, the burner being mounted togetherwith the engine and heat exchanger.
 13. An assembly according to claim10, wherein the Stirling engine is mounted with its head lowermost andthe heat exchanger directly beneath the head.
 14. An assembly accordingto claim 10, further comprising a supplementary burner mounted togetherwith the engine and heat exchanger to supply additional heat to the heatexchanger.
 15. An assembly according to claim 10, wherein the heatexchanger comprises a helical coil centred on the main axis of theengine.
 16. An assembly according to claim 15, wherein the heatexchanger has a main axis which is coaxial with an axis about which thehelical coil is wound.
 17. A system according to claim 1, wherein theheat exchanger comprises a helical coil wound around an axis and whichextends along the full axial length of the supplementary burner.